1. Field of the Invention
This invention relates to a device to indicate an unacceptable pressure differential across the filter in a liquid circuit, such as a lubricating oil circuit or an hydraulic circuit. More particularly, this invention relates to a pressure differential indicator that has a nonmechanical temperature sensitive lockout feature to prevent the device from giving an excessive pressure differential condition indication when such condition exists by virtue of the viscosity of the circulating fluid at low temperatures, as opposed to a clogged condition of the filters.
2. Description of the Prior Art
It is known that it is useful to use a pressure differential indicating device in conjunction with a filter in a lubricating system or an hydraulic system to indicate a clogged condition of the filter, as evidenced by an excessive pressure differential across the filter, and it is known that it is useful to provide such a device with a temperature sensitive feature which will prevent the device from indicating a clogged condition when the pressure differential across the device is due to a low fluid temperature and the high fluid viscosity which results therefrom. For example, U.S. Pat. No. 3,117,550 (Cole) discloses a temperature sensitive pressure differential indicator in which the actuation of an indicator pin into a high pressure differential indicating position is effected by the springing of a temperature-compensating bimetallic convex disk; and U.S. Pat. No. 4,172,971 (Silverwater, et al) discloses a temperature sensitive pressure differential indicator which utilizes axially aligned magnetic elements in the actuation of the high pressure differential indicating button, with a thermostatic element to physically decouple the system at low operating temperatures to prevent a false indication of a high pressure differential at such low temperatures. Other prior patents which disclose the use of bimetallic temperature lockout elements in a liquid filter excessive pressure differential indicator are U.S. Pat. Nos. 3,812,816 (Juhasz); 3,386,410 (Barnes, Jr.); 3,402,690 (Willis); 3,140,690 (Siebel); and 2,942,572 (Pall).
Prior art devices of the type described above have certain problems associated with their use, however. One of the problems is that they rely on mechanical interference between parts that are movable relative to one another in response to a change in temperature to perform the thermal lockout feature, and the movement of such parts relative to one another, and the loads between them when they are in mechanically interferring position relative to one another lead to wear on such parts and can lead to eventual failure of one or another of such parts or loss of reliability in their operation. Additionally, the reliance on mechanical interference between parts that are movable relative to one another for the performance of a thermal lockout feature, as is characteristic of the above-described prior art devices, can lead to unreliable performance of such devices in an environment where they are subject to high frequency vibrations or shock loads of substantial magnitude.